In digital laser printers, it is common to employ lasers to write on photosensitive media such as silver halide film or laser thermal dye media. The laser emits a beam which must be modulated by some means in order to write each line of a raster image. It is common practice to use an acousto-optic modulator (hereinafter referred to as an AOM) for this purpose. An AOM might typically be positioned in the path of a laser beam which has been focussed by a lens. In a polychromatic laser printer, an AOM is required for each wavelength in order to independently modulate each color. In one method of laser printing, the multiple wavelength-beams are combined together into a "white" beam which is then shaped for scanning with a spinning polygon through a focussing lens onto the photosensitive medium. One method for combining the multiple wavelengths is a fiber optic multiplexer as described in U.S. Pat. No. 5,666,447. The input to the fiber optic multiplexer is a single mode optical fiber for each color. In order to couple most of the optical power into such a fiber, it is necessary to focus the beam to a very small spot, typically 0.004 mm in diameter. This spot must be positioned with less than 0.001 mm and accurately aligned in its propagation direction.
In such a configuration, the relative positions of the laser, its focussing lens, the AOM, the fiber coupling lens and the end of the glass fiber must all be very precisely adjusted. The alignment of all these components is very critical, and must be very carefully maintained if the maximum amount of the laser's power is to be injected into the fiber. This alignment is preferably done in the factory under laboratory conditions. There is a considerable amount of apparatus that is used to make the small precise adjustments and also to detect the position and power of the laser beam as it travels through the system. It would be quite inconvenient, if not difficult to set these alignments in the field.
In the event of the failure of a laser (in the field), a new laser would need to be installed in place of the failed one. But the dimensions and optical characteristics of the lasers are not accurate enough to allow the new laser to be installed without adjustment. Thus a failed laser implies the need for an optical realignment of the replacement laser, its lens, and the AOM relative to the fiber coupler. As previously explained, this presents a difficulty, as field adjustment of these critical components is not practical.
Laser printer products for photosensitive media in a visible wavelength range such as disclosed in Firth et al, "A Continuous Tone Laser Color Printer" Journal of Imaging Technology 14:78-89 (1988) have been known for years. Until recently, the laser choices for exposing such media in the green and blue wavelengths has been restricted due to power, wavelengths available, and stability. Argon lasers have often been used, but these lasers are noisy, large and require bulky air cooling with limited lifetimes. Now blue and green solid state lasers are available in much more compact packages without the need for air cooling. Because these lasers are new, their reliability and lifetime are not as high as desired.
Optical systems are normally aligned to the laser beam one component at a time starting near the laser. Each component is then securely positioned. Most systems have very tight tolerances on beam position so that they cannot tolerate even small laser beam position and pointing changes. Lasers have too much variability in their beam sizes, beam positions and angular pointing with respect to the external laser package to allow a simple swapping of lasers. This means that a laser failure, as noted above, is a major problem requiring a replacement of the whole laser printer product due to the impracticability of replacing and aligning such a complex optical system to a new laser outside the factory.
In a color laser printer using three lasers in the primary color wavelengths, an AOM is required for each laser to independently modulate the exposure of each color in order to obtain full color images with a wide gamut. Over a long time period of use, however, the AOM can deteriorate and fail to provide a sufficiently high modulation level and transmission of optical power. AOMs have variability in performance due in part to internal crystal positional and angular tolerances with respect to their packages. In building the optical system for a laser printer, AOMs are placed near the beginning of the train and subsequent components are carefully aligned to the laser beam diffracted by the acoustic wave in the crystal. An AOM failure is thus also a major problem and is impractical to replace in a product outside the factory.
In order to minimize maintenance costs of a laser printer product over several years it is therefore highly desirable to have a laser printer in which the laser and AOM can be replaced by a service person without having to perform extensive alignment of the optical system outside the factory.